Drill bit

ABSTRACT

The present invention relates generally to drill bits used in drilling subterranean boreholes. More specifically, the invention relates to drill bits and cutting elements on the drill bits and the design of each. The invention utilizes a cutting element having a convex curved top portion with a shear face wherein said shear face is a plane formed by taking a planer slice from said convex curved top portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/794,943 filed Mar. 15, 2013, which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to drill bits used in drilling subterranean boreholes. More specifically, the invention relates to drill bits and cutting elements on the drill bits and the design of each.

BACKGROUND OF THE INVENTION

In a typical drilling operation, a drill bit is advanced into a soil or rock formation. The drill bit may chip and cut away rock to produce a borehole through rotary action, percussive action or both rotary and percussive action. Often to enhance drilling, especially through harder rock and prior equipment used downhole, the drill bit will utilize inserts or cutting elements. Conventional cutting inserts typically have a body consisting of a cylindrical grip or “post” portion from which a convex cutting end extends. In order to improve their operational life, these inserts are sometimes coated with a superhard material, sometimes also known as an ultrahard material. The coated cutting layer typically comprises a superhard material, such as a layer of polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN). The substrate, which supports the cutting layer, is normally formed of a hard material such as tungsten carbide (WC). The grip is embedded in and affixed to the drill bit and the cutting end extends outwardly from the surface of the drill bit. The cutting end may be hemispherical, which is commonly referred to as a semi-round top (SRT).

In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. This process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits that will drill faster and longer and that are usable over a wide range of formation hardnesses.

Accordingly, although cutting elements have significantly extended the life of drill bits and expanded the scope of formations for which drilling is economically viable, there is ever a need for improving the performance and durability of cutting elements to enhance the operation of drill bits.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, there is provided a cutting element comprising a substrate column comprising a first material. The substrate column has a convex curved top portion with a shear face, wherein the shear face is a plane formed by taking a planer slice from the convex curved top portion. An ultrahard material layer is disposed on the convex top portion.

In accordance with another embodiment of the invention there is provided a drill bit comprised of a drill head having a plurality of cutting elements thereon. The cutting elements comprising a substrate column comprising a first material. The substrate column has a convex curved top portion with a shear face, wherein the shear face is a plane formed by taking a planer slice from the hemispherical top portion. An ultrahard material layer is disposed on the convex curved top portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective view of a cutting element in accordance with an embodiment of the present invention.

FIG. 2 is side cutaway view of a cutting element in accordance with the embodiment of FIG. 1.

FIG. 3 is a side perspective view of a cutting element in accordance with the embodiment of FIG. 1. FIG. 3 shows exemplary dimensions for the cutting element.

FIG. 4 is a cutaway of an embodiment of a cutting element in accordance with another embodiment of the present invention.

FIG. 5 is a partial perspective view of a rotary-percussive drill bit in accordance with the present invention.

FIG. 6 is a side perspective view of a blade or drag bit, also referred to as a PDC drill bit, in accordance with the present invention.

DETAILED DESCRIPTION

Referring now to the drawings, wherein like reference numbers are used herein to designate like elements throughout the various views, embodiments of the present invention are illustrated and described, and other possible embodiments of the present invention are described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated and/or simplified in places for illustrative purposes only. One of ordinary skill in the art will appreciate the many possible applications and variations of the present invention based on the following examples of possible embodiments of the present invention.

Referring now to FIG. 1, a cutting element 10 in accordance with an embodiment of the invention is illustrated. Cutting element 10 is a drill bit insert designed both for percussive or impact chipping and for cutting or shearing of subterranean material, which can be rock, soil, or structures and materials previously placed in the borehole such as a casing or shoe. Cutting element 10 includes a substrate column 12 and an ultrahard layer 20 formed on a top end 16 of substrate column 12. Substrate column 12 comprises a base or grip 14 having outer surface 18, and a top end 16 having an outer surface 22. Substrate column 12 is formed from a material, typically a carbide material. Generally, the material is metal carbide such as tungsten carbide and can be formed by a cementing process. Accordingly, powdered metal carbide can be provided in a mold with a metal binder and heated under pressure to cause the binder to infiltrate and cement the metal carbide powder into a substrate body having the desired interface surface geometry. Alternatively, the desired surface geometry may be machined on the substrate column.

Base or grip 14 is designed to mount into a drill bit and provided adequate elevation to top end 16 so that top end 16 extends out from the drill bit surface and contacts the subterranean material. Base 14 may be characterized as being a “post” shape and is, generally, a cylindrical—or disc-shaped body; however, other shapes can be utilized.

Top end 16 is a convex curve in shape. Top end 16 can be a convex shape having a simple curve such as oval, hemisphere, or ballistic shape. FIGS. 1-3 illustrate an embodiment where cutting element 10 has a top end 16 with a hemispherical shape. As used herein, the term “hemispherical” means true hemispheres (half of a sphere) and partial hemispheres or spherical caps.

As can be seen from FIGS. 1 and 2, top end 16 has a planer slice removed so as to form shear face 24, which is a planer section of top end 16. Shear face 24 is generally circular or oval in shape with circular being currently preferred. As can be more clearly seen from FIG. 2, shear face 24 is at an angle to longitudinal axis 26, as measured by inner angle α. Generally, shear face 24 is at an angle of less than 80°. For hemispherical top ends, inner angle α can typically be at an angle of from about 40° to about 80° with the longitudinal axis, and can be from 50° to 70°. As shown, in FIG. 2 angle α is 60°.

Generally, shear face 24 will be above interface 28 so as to leave a curved edge 30 between shear face 24 and interface 28. Curved edge 30 will have a width 31, as measured at its narrowest portion from interface 28. Generally, width 31 can be at least 5% of the surface length from interface 28 to tip 27 and can be at least 10% or at least 15% of such surface length. Typically, width 31 can be no more than 30% of the surface length from interface 28 to tip 27 but can be no more than 25% or no more than 20% of such surface length.

In general, shear face 24 has a radius RS less than 50% of the surface length of the top end from interface 28 to tip 27 to allow for curved edge 30. Generally, radius RS is greater than 10% of the surface length from interface 28 to tip 27, and can be from 25% to 45%. Where top end 16 is a hemisphere, shear face 24 has a radius RS that is about 50% or less than the radius Ri, which is the radius of the top end 16 at interface 28. Also, width 31 generally can be at least 5% of radius Ri.

Turning now to FIG. 3, exemplary dimensions for one embodiment of a cutting element in accordance with the invention is shown. The embodiment of FIG. 3 has a hemispheric top end 16 in accordance with the embodiment described above for FIGS. 1 and 2. The dimensions illustrated are based on percentages of the height of cutting element 10. The height of the insert from the bottom of the insert to where the dome curvature begins is represented by k. The height from the bottom of the insert to the center point of shear face 24 is represented by l. The distance between longitudinal axis 26 and the center of shear face 26 is represented by m. In the exemplary embodiment of FIG. 3, k is equal to 0.42, l is equal to 0.63 and m is equal to 0.12 of the height to cutting element 10. Accordingly, if inner angle α is equal to 60° then RS would be equal to 0.20 of the height of cutting element 10.

An ultrahard material layer 20 is disposed on top end 16. Generally, at least all of the convex portion of top end 16 will be covered with the ultrahard material so that, below interface 28, the cutting element will have an outer surface 18 that is substrate material and, above interface 28, the cutting element will have an outer surface 22 that is ultrahard material. Shear face 24 can have an outer surface that is substrate material or can have ultrahard material deposited thereon so that outer surface is ultrahard material.

The ultrahard material forming layer 20 can be a polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or other suitable hard crystalline material. PCD is presently preferred as the ultrahard material. The ultrahard material can be deposited by methods known in the art. Typically, there will be a transition layer between ultrahard material 20 and the substrate material. In one exemplary method of making the cutting element, diamond powder is positioned adjacent to carbide substrate in a pre-shaped can, which is of the shape of the final cutting tool without shear face 24. The diamond powder is positioned in that portion of the pre-shaped can that will form the top end of the cutting element. The pre-shaped can is sintered under high pressure and high temperature to form the cutting element. The pre-shaped can is removed and the top end 16 is machined to form shear face 24. The resulting cutting element has no PCD on shear face 24.

Turning now to FIG. 4, another embodiment of the invention is illustrated in which a cutting element 48 has a the top end 16 having a ballistic shape. As used herein “ballistic” means a curve having a generally bullet shape; that is elongated in the direction of longitudinal axis of the cutting element. A ballistic top end 16 generally has a side 52 having an arcuate cross-section, which lies on a circle having a radius R1. Additionally ballistic top 16 has top spherical cap 50 in tangential relationship with side 52. Spherical cap 50 has a radius less than R1 and as a width across opposing tangential points 54 and 56 of R2. In one embodiment of the invention the ratio of R1 to R2 is from 4 to 24 and can be 4 to 10. Additionally, top end 16 has a height H and a radius at interface 28 of Ri.

Similar to the embodiment illustrated in FIGS. 1-3 above, shear face 24 of the embodiment of FIG. 4 will be above interface 28 so as to leave a curved edge 30 between shear face 24 and interface 28. Curved edge 30 will have a width 31, as measured at its narrowest portion from interface 28. Generally, width 31 can be at least 5% of the surface length from interface 28 to tip 27 and can be at least 10% or at least 15% of such surface length. Typically, width 31 can be no more than 30% of the surface length from interface 28 to tip 27 but can be no more than 25% or no more than 20% of such surface length.

In general, shear face 24 has a radius RS less than 50% of the surface length of the top end 16 from interface 28 to tip 27 to allow for curved edge 30. Generally, radius RS is greater than 10% of the surface length from interface 28 to tip 27, and can be from 25% to 45%. Where top end 16 is a ballistic shape, shear face 24 has a radius RS that can be about 50% or less of the radius R1. Generally, radius RS can be from 25% to 50% of radius R1. As indicated above, shear face 24 will be above interface 28 so as to leave a curved edge 30 between shear face 24 and interface 28. Where top end 16 is a ballistic shape, curved edge 30 will have a width 31, at its narrowest portion. Generally, width 31 can be at least 5% of radius R1.

The inventive cutting element can be used in a variety of drill systems for drilling into subterranean formations. Thus, it is useful in rotary-percussive drill bits, PDC drill bits and fixed cutter bits. While applicable to a variety of drill bits, it is especially useful in rotary-percussive drill bits for the reasons explained below.

Turning to FIG. 5 a drill bit 32 a rotary-percussive drill bit utilizing cutting elements 10 is shown. As the name indicates, rotary-percussive drill bits use both rotary and percussive action in order to chip away rock and produce a hole. The combination of rotation and percussion helps the drill achieve a cutting and grinding action at the same time as a chipping action. A hole is formed when energy is transmitted pneumatically through the drill rod to the drill bit prompting it to thrust into the formation using a repeated hammering motion. The impact of the percussion component is enough to break and dislodge rock, which subsequently removes debris and cuttings with compressed air or water flow. The rotational component of the drill occurs as the drill is producing percussion strikes against the formation

Drill bit 32 comprises drilling head 34 having a plurality of cutting elements 10 thereon. The cutting elements 10 are in accordance with the above description and are mounted in pockets 44. Base 14 of cutting element 10 is inserted into pockets 44, typically by pressing with a press. When a cutting elements 10 is mounted in drill bit 32, base 14 is in pocket 44 and top end 16 extends out from pocket 44 so as to be able to interact with subterranean material during drilling.

Drill bit 32 has a longitudinal axis 36 and includes radially innermost impact surfaces 38 and 39, which are generally transverse to longitudinal axis 36 though, as shown, impact surface 38 can be concave, and impact surface 39 can annular and frustoconical so as to be slightly angled to transverse. Drill bit 32 includes a radially outermost gage surface 40, which has a generally frustoconical annular shape. Moving radially inward from gage surface 40, drill bit 32 includes an annular, generally frustoconical inner surface 41. Gage surface 40 extends from inner surface 41 to skirt surface 42, which is annular and generally frustoconical but angles oppositely from gage surface 40 so as to form edge or shoulder 43. Gage surface 40 is generally at an inner angle β of from 35° to 42° degrees from longitudinal axis 36. Cutting elements 10 can be mounted on impact surfaces 38 and 39, inner surface 41, gage surface 40 and skirt surface 42. Alternatively, cutting elements 10 can be mounted on only gage surface 40, and skirt surface 42 and impact surface 38 can use a traditional impact cutting element, such as one having a hemispherical top end without a shear surface. Cutting elements 10 incorporated on skirt surface 42 and can be truncated to provide shearing action to the bore wall and, thus provide a smoother hole. Additionally, cutting elements 10 can be mounted on only gage surface 40, with the other surfaces using traditional impact cutting elements.

The use of cutting elements 10 on gage surface 40 provide for one or more axial cutting elements for repeated axial impacts. Additionally, shear face 24 provides for shearing action. Thus, the above drill bit 32 allows the cutting elements 10 to absorb repeated impact loading of air hammers, and at the same time provide a shearing action of subterranean materials in response to rotary motion. The amount of deeper penetration that is desired depends on the hardness and type of subterranean material into which the borehole is being drilled. Harder rock formations require deeper penetration of the axial cutters. As the cutter elements 10 are rotated, the shear face 24 provides a shearing action to the rock thereby making the borehole smoother.

It is not uncommon for diamond coated inserts to fail during cutting. Failure typically takes one of three common forms, namely spalling/chipping, delamination, and wear. External loads due to contact tend to cause failures such as fracture, spalling, and chipping of the diamond layer. The impact mechanism involves the sudden propagation of a surface crack or internal flaw initiated on the PCD layer, into the material below the PCD layer until the crack length is sufficient for spalling, chipping, or catastrophic failure of the enhanced insert. On the other hand, stresses resulting from manufacturing processes tend to cause delamination of the diamond layer, either by cracks initiating along the interface and propagating outward, or by cracks initiating in the diamond layer surface and propagating catastrophically along the interface. The inventive cutting elements incorporating a shear face have been demonstrated to be stronger than traditional cutting elements using a hemispherical top end and no shear face. For example, the shear face acts as a stress relief when the cutting element is pressed into the drill bit pockets; thus, reducing the deleterious effects of stress during production of the drill bit. Accordingly, the inventive cutting elements are more resistant to spalling/chipping, delamination, and wear than traditional cutting elements and the shear face not only provides for improved shearing action of the cutting elements, but for a stronger cutting element having a longer life.

Turning now to FIG. 6, the use of the inventive cutting element on a blade or drag bit 60, sometimes referred to as a PDC drill bit, is illustrated. A PDC drill bit uses polycrystalline diamond compact cutters to shear rock with a continuous scraping motion. As indicated above, the inventive cutting elements have improved shearing action and, thus, are exceptional for use in PDC drill bit applications. Drill bit 60 is a blade or drag bit utilizing the cutting elements 10. Drill bit 60 comprises drilling head 62 having a plurality of blades 64 with a plurality of cutting elements 10 thereon. The cutting elements 10 are in accordance with the above description and are mounted in pockets 66. Base 14 of cutting element 10 is inserted into pockets 66, typically by pressing with a press. When cutting elements 10 are mounted in drill bit 60, base 14 is in pocket 66 and top end 16 extends out from pocket 66 so as to be able to interact with subterranean material during drilling.

Other embodiments of the current invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Thus, the foregoing specification is considered merely exemplary of the current invention with the true scope thereof being defined by the following claims. 

What is claimed is:
 1. A cutting element for use in a drill bit for drilling a borehole in earthen formations, the cutting element comprising: a substrate column comprising a first material, said substrate column having a convex curved top portion with a shear face wherein said shear face is a plane formed by taking a planer slice from said convex curved top portion; an ultrahard material layer disposed on said hemispherical top portion.
 2. The cutting element of claim 1 wherein said substrate column has a longitudinal axis and said shear face is at an inner angle of less than 80° from said longitudinal axis.
 3. The cutting element of claim 1 wherein said shear face is comprised of said first material without any ultrahard material disposed thereon.
 4. The cutting element of claim 1 wherein said shear face has said ultrahard material disposed thereon.
 5. The cutting element of claim 1 wherein said convex curved top portion has a shape of either hemispheric or ballistic.
 6. The cutting element of claim 5 wherein said shape of said convex curved top portion is hemispheric.
 7. The cutting element of claim 6 wherein said substrate column has a longitudinal axis and said shear face is at an inner angle of from 40° to 80° from said longitudinal axis.
 8. The cutting element of claim 7 wherein said inner angle is from 50° to 70°.
 9. A drill bit for drilling a borehole in earthen formations, the drill bit comprising: a drill head having a plurality of cutting elements thereon, said cutting elements comprising: a substrate column comprising a first material and having a convex curved top portion with a shear face, wherein said shear face is a plane formed by taking a planer slice from said convex curved top portion; and an ultrahard material layer disposed on said hemispherical top portion.
 10. The drill bit of claim 9 wherein said drill bit is a rotary percussive drill bit.
 11. The drill bit of claim 5 wherein said drill bit further comprises a longitudinal axis, an impact surface transverse to said longitudinal axis and a gage surface having an angle of from 35° to 42° degrees from said longitudinal axis, wherein said gage surface has a plurality of said cutting elements mounted thereon and configured such that upon use said cutting elements provide percussive and shear drilling functions.
 12. The cutting element of claim 9 wherein said substrate column has a longitudinal axis and said shear face is at an inner angle of less than 80° from said longitudinal axis.
 13. The cutting element of claim 9 wherein said shear face is comprised of said first material without any ultrahard material disposed thereon.
 14. The cutting element of claim 9 wherein said shear face has said ultrahard material disposed thereon.
 15. The cutting element of claim 9 wherein said convex curved top portion has a shape of either hemispheric or ballistic.
 16. The cutting element of claim 9 wherein said shape of said convex curved top portion is hemispheric.
 17. The cutting element of claim 16 wherein said substrate column has a longitudinal axis and said shear face is at an inner angle of from 40° to 80° from said longitudinal axis.
 18. The cutting element of claim 17 wherein said inner angle is from 50° to 70°. 